How to add this particular voice

Hello,
Well, actually I don't really know whether this is a voice or not. I do know how to add a voice, yet I don't know how to write this particular "structure".
I will post a photo so it will be more clear to you.

The composition is in 4/4 ..the first bar, as you can see, is already composed by a 4/4 chord so A and Bb is added , isn't it?
How can I add that??

Thank you